1. Field of the Invention
The invention relates to hybrid vehicle powertrains in which an electric motor and an internal combustion engine provide power to a transmission having no torque converter.
2. Description of the Prior Art
In a automotive vehicle powertrain that includes a transmission, a hydrokinetic torque converter is conventionally located between the engine shaft and transmission input, the torque converter increasing engine torque during launch to provides a powerful launch feel to the driver during acceleration from a stop. The impeller of the torque converter receives engine torque and the turbine of the torque converter transfers torque to the torque input element of multiple-ratio gearing of the transmission.
The presence of the torque converter in the torque flow path introduces hydrokinetic power losses, particularly during startup of the vehicle, as the torque converter fluid in the converter torus circuit is accelerated and decelerated. These losses are manifested in heat loss to the hydrokinetic fluid, which requires a heat exchanger to maintain an acceptable hydrokinetic fluid temperature.
Attempts have been made to eliminate the power losses inherent in a powertrain having an internal combustion engine and a torque converter automatic transmission by replacing the engine with an electric motor and by replacing the torque converter by a clutch. The power output of the motor is connected to the clutch, which is connected to the transmission input. However, replacing the torque converter in this way has met with limited success because of the lack of acceleration at launch, i.e. when accelerating the vehicle from a stop.
Attempts to combine the advantages of an internal combustion engine with an electric motor drive have been made in hybrid vehicle arrangements, but the engine is required in such known designs to operate throughout a large speed range including startup speeds and to operate at idle speed while the vehicle is at rest.
In a hybrid gas-electric vehicle with a pre-transmission motor, it is highly desirable to eliminate the torque converter to minimize transmission losses. An electric torque convertless transmission has been designed to address drivability concerns while minimizing losses. That transmission uses a base power-shifting transmission with standard gear ratios, removes the torque converter, and places a high voltage motor on the transmission input. Without the torque converter, the launch for the transmission is accomplished by actively controlling the existing planetary clutches and the electric motor. A control system for coordinating the clutch and motor to provide acceptable launch feel is required.